Thermally synthesized nanosized copper ferrites as catalysts for environment protection

Catalysis Communications 12 (2010) 105–109

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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m

Thermally synthesized nanosized copper ferrites as catalysts for environment protection T. Tsoncheva a,⁎, E. Manova b, N. Velinov b, D. Paneva b, M. Popova a, B. Kunev b, K. Tenchev b, I. Mitov b a b

Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 9, 1113 Sofia, Bulgaria Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 11, 1113 Sofia, Bulgaria

a r t i c l e

i n f o

Article history: Received 19 May 2010 Received in revised form 28 July 2010 Accepted 4 August 2010 Available online 26 August 2010 Keywords: Copper ferrites Thermal synthesis Mössbauer spectroscopy Methanol decomposition Toluene oxidation

a b s t r a c t Nanosized copper ferrites were prepared by thermal method from the corresponding hydroxide carbonate precursors varying the temperature of synthesis. The phase composition of the obtained materials was characterized by XRD, Mössbauer spectroscopy, DSC and TPR analysis. Their catalytic properties were tested in total oxidation of toluene and methanol decomposition to CO and hydrogen. The relation between the synthesis parameters, phase composition of the samples and their transformation under the oxidation or reduction reaction medium was investigated in turns to understand the catalytic behavior of the obtained materials. © 2010 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

During the last decade spinel ferrites receive enormous attention due to their importance as magnetic materials, semiconductors, and pigments [1–3], as well as effective catalysts for number of catalytic processes such as methane reforming, treatment of automotive exhausted gases, hydrogen production by steam reforming of DME [4–10] etc. It has been established that the crystal symmetry and the properties of these materials are highly sensitive to the cation distribution in the spinel ferrite lattice that depends on the preparation method [4,11–14]. The aim of the present paper is to study the catalytic behavior of thermally obtained nanosized copper ferrites in total oxidation of toluene and methanol decomposition to CO and hydrogen. Recently, these catalytic reactions have been widely studied due to their ecological importance for VOCs elimination in toxic gas emissions [15,16] and as a source of alternative clean and efficient fuel [17,18], respectively. Special attention is paid on the relation between the catalytic performance of the obtained materials and their final phase composition, determined both by the changes of the synthesis parameters and by the influence of the oxidation or reduction medium, which realizes during the studied catalytic reactions.

Copper ferrites were prepared by thermal synthesis using coprecipitated copper and iron hydroxide carbonates as precursors. The starting solution of Fe(NO3)3.9H2O and Cu(NO3)2.3H2O in molar ratio of 2:1 was precipitated with drop wise addition of 1 M Na2CO3 up to pH 9 at continuous stirring. The obtained precipitates were dried at room temperature to form the precursor powder, denoted as CuFe(HC), which was further heated at 573, 673, 773, 973 and 1073 K. The samples were named as CuFe(T), where T is the annealing temperature in K. The powder XRD patterns were recorded by use of a TUR M62 diffractometer with Co Kα radiation. The observed patterns were cross-matched with those in the JCPDS database. The room (RT) and liquid nitrogen temperature (LNT) Mössbauer spectra were obtained with a Wissel (Wissenschaftliche Elektronik GmbH, Germany) electromechanical spectrometer working in a constant acceleration mode. A 57Co/Cr (activity ≅ 10 mCi) source and α-Fe standard were used. The parameters of hyperfine interaction such as isomer shift (IS), quadrupole splitting (QS), effective internal magnetic field (Heff), line widths (FWHM), and relative weight (G) of the partial components in the spectra were determined. Temperatureprogrammed reduction (TPR) of the samples was carried out in the measurement cell of a differential scanning calorimeter (DSC-111, SETARAM) directly connected to a gas chromatograph (GC). Measurements were made in the 300–973 K range at 10 K/min heating rate in a flow of Ar:H2 = 9:1, the total flow rate being 20 ml/min. A cooling trap between DSC and GC removes the water obtained during the reduction. The simultaneous Thermogravimetry-Differential Scanning

⁎ Corresponding author. E-mail address: [email protected] (T. Tsoncheva). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.08.007

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Calorimetry was carried out by Linseis STA1600 thermobalance in static air at 10 K/min heating rate. Methanol decomposition experiments were carried out in a flow reactor using argon as a carrier gas, at methanol partial pressure of 1.57 kPa and WHSV-1.5 h−1. On-line gas chromatographic analyses were performed on HP 5980 with PLOT Q column, with simultaneous using detector of thermo conductivity and flame ionization detector and an absolute calibration method. Toluene oxidation was studied in a flow reactor using air as carrier gas, at toluene partial pressure of 0.9 kPa and WHSV-1.2 h− 1. On-line gas chromatographic analyses were performed on HP 5980 with PLOT Q column using detector of thermo conductivity. 3. Results and discussion Similarly to the hydroxide carbonate precursor (HC), XRD patterns of the materials obtained at annealing temperature below 773 K represent only a halo of almost amorphous material (Fig. 1). Well defined reflections, typical of crystalline CuFe2O4 phase, are registered

for the samples that were synthesized above this temperature. Generally, the obtained product (Table 1) represents a mixture of ferrites with cubic and tetragonal symmetry [12,13], with a trend to increase of the relative part of the latter at higher temperatures. According to [12,19] the observed distortion in cubic structure with the appearance of tetragonal one is due to the higher population of Cu ions in octahedral positions, providing a cooperative Jahn–Teller effect. Presence of α-Fe2O3 is also found for CuFe(773) and CuFe(973). The average particles size and the microstrain degree are determined by the Williamson–Hall method [14]. According to the method, the diffraction lines are treated using the Voigt function, which Lorentzian and Gaussian components are used for the elucidation of the average particles size and the mean level of the internal strain, respectively (Table 1). An increase of the degree of crystallization and agglomeration of the spinel nanoparticles with the annealing temperature increase is observed, the effect being most intensive at about 773 K. These data are in agreement with DSC profile of CuFe(HC) precursor (ESI), where a significant exothermal effect with a maximum at 798 K, associated with the crystallization of CuFe2O4 phase, is observed.

PDF 34-0425 (CuFe2O4 - tetragonal) 5000

PDF 25-283(CuFe2O4 - cubic)

4000

PDF 79-1741 (Hematite)

3000

CuFe(1073)

2000 1000 0 5000 4000 3000

CuFe(973)

2000 1000 0 5000 4000

Intensity, counts

3000

CuFe(773)

2000 1000 0 1600 1400 1200

CuFe(673)

1000 800 600 400 1600 1400

CuFe(573)

1200 1000 800 600 400 1600 1400

CuFe(HC)

1200 1000 800 600 400 6

5

4

3

2

d, A

Fig. 1. XRD patterns of hydroxide carbonate precursor (HC) and the obtained materials under different annealing temperatures.

T. Tsoncheva et al. / Catalysis Communications 12 (2010) 105–109 Table 1 Average crystallites size (D), degree of microstrain (e) and lattice parameters (a, c) determined from XRD patterns. Sample

Phase

D, nm

e * 103, a.u.

a, Å

c, Å

%

CuFe(773)

CuFe2O4-cubic CuFe2O4-tetragonal α-Fe2O3 CuFe2O4-cubic CuFe2O4-tetragonal α-Fe2O3 CuFe2O4-cubic CuFe2O4-tetragonal

15.68 20.64 19.21 15.10 29.13 23.33 13.55 27.01

3.11 2.55 2.41 2.77 1.91 1.78 4.66 1.73

8.38 5.82 5.05 8.38 5.81 5.00 8.39 5.81

− 8.63 13.67 − 8.68 13.78 − 8.68

20 67 13 8 88 4 10 90

CuFe(1073)

Table 2 Parameters of Mössbauer spectra of initial samples. Sample

Components

IS, mm/s

QS, mm/s

Heff, T

FWHM, mm/s

G, %

CuFe(HC) CuFe(573)

Db–Fe3+octa Db1–Fe3+octa Db2–Fe3+octa Db1–Fe3+octa Db2–Fe3+octa Db1–Fe3+octa Db2–Fe3+octa Sx1–CuFe2O4–Fe3+octa Sx2–CuFe2O4–Fe3+tetra Sx3–α-Fe2O3–Fe3+octa Sx1–CuFe2O4–Fe3+octa Sx2–CuFe2O4–Fe3+tetra Sx3–α-Fe2O3–Fe3+octa Sx1–CuFe2O4–Fe3+octa Sx2–CuFe2O4–Fe3+tetra

0.36 0.34 0.32 0.34 0.33 0.43 0.42 0.37 0.27 0.38 0.36 0.26 0.37 0.36 0.26

0.68 0.62 1.09 0.61 1.16 0.64 1.15 −0.11 −0.02 −0.10 −0.13 −0.01 −0.10 −0.14 −0.01

– – – – – – – 50.7 47.6 51.3 51.7 48.7 51.3 51.5 48.7

0.44 0.40 0.48 0.45 0.52 0.50 0.56 0.50 0.59 0.40 0.55 0.49 0.40 0.54 0.51

100 48 52 48 52 49 51 33 63 4 50 46 4 49 51

CuFe(673) CuFe(673)_LNT CuFe(773)

RT Mössbauer spectra of the studied materials are presented in Fig. 2 and the corresponding parameters are listed in Table 2. The spectra of the materials obtained below 773 K are well fitted with two doublets with parameters typical of Fe3+ ions in octahedral coordination. We assign these peaks to the presence of finely dispersed iron containing particles with average size below 10–12 nm. The preservation of the doublet character of LNT spectra for these samples is an evidence that the particles size is predominantly about 3–4 nm (Fig. 2, Table 2). The different values of QS for the samples obtained below 773 K in comparison with that one of HC precursor point to the changes in the Fe3+ ions environment, probably due to the partial decomposition of the precursor and formation of new oxide phase that is in agreement with the data obtained by DSC analysis (see ESI). On the contrary, RT Mössbauer spectra of the samples obtained above 773 K (Fig. 2, Table 2) consist of two sextets with hyperfine field and isomer shifting values typical of octahedrally and tetrahedrally coordinated Fe3+ ions in copper ferrite, [11 and refs. therein] and this result is in agreement with XRD and DSC data (see above). Third sextet component with relative part about 4% and

CuFe(973)

CuFe(1073)

parameters corresponding to Fe3+ ions in hematite nanoparticles, appears in the Möessbauer spectra of CuFe(773) and CuFe(973) materials. The TPR profiles of the samples are illustrated in Fig. 3. In accordance with DSC analysis (ESI), XRD (Fig. 1, Table 1) and Mössbauer spectra (Fig. 2, Table 2), the low temperature effect with a maximum at about 440 K, which is observed only for the materials synthesized below 773 K, is probably due to the partial decomposition of HC precursor and/or due to the reduction of finely dispersed copper oxide species, which were not still organised in crystalline ferrite spinel structure. However, two general effects with maximum in the range of 450–600 K and 800–860 K are distinguished for all materials, indicating the step-wise reduction of copper ferrite to Cu and magnetite and to metallic iron, respectively [8,10,20]. The shift of

100

100

98 98

96

CuFe(HC)

94

96

92 90

94

CuFe(773)

88 92

86

Relative transmission, %

-10

-5

0

5

10

100

98

CuFe(673) 96

94

-10 100 98 96 94 92 90 88 86 84 82 80 -10

-5

0

5

10

-10

Relative transmission, %

CuFe(973)

107

-5

0

5

10

5

10

5

10

100 98 96 94

CuFe(973) 92 -10

-5

0

100

98

CuFe(673)_LNT

96

CuFe(1073)

-5

0

Velocity, mm/s

5

10

94 -10

-5

0

Velocity, mm/s

Fig. 2. RT and LNT Möessbauer spectra of selected materials before catalytic tests.

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T. Tsoncheva et al. / Catalysis Communications 12 (2010) 105–109

CuFe(1073)

Consumption of H2, a.u.

CuFe(973)

CuFe(773)

CuFe(673)

CuFe(573)

400

600

800

Temperature, K Fig. 3. TPR profiles of various thermally synthesized materials.

the TPR profiles of CuFe(973) and CuFe(1073) samples to higher temperatures is ascribed to the transformation of cubic ferrite structure to tetragonal one and/or to the particle size increase (Table 1, Fig. 1). The temperature dependencies of total oxidation of toluene to CO2 for various materials are presented in Fig. 4. The catalytic activity is registered above 500 K and no products of partial toluene oxidation are found. The conversion curves are clearly shifted to higher temperatures with the increase of the annealing temperature of ferrites synthesis. Mössbauer measurements (Table 3, ESI) reveal that under the reaction medium only further crystallization without any significant phase transformation of the obtained ferrites occurs. However, the changes in the catalytic activity of the samples well correlate with the changes in their reduction ability (Fig. 3). This

result is not surprising in a view of the data reported in the literature [21], where Mars van Krevelen mechanism, including the release of oxygen from the catalyst lattice, is generally discussed. We assign the observed effects to the transformation of cubic spinel ferrite to tetragonal one as well to the particles size increase with the annealing temperature increase (Figs. 1, 2; Tables 1, 2). The profile of the conversion curves of methanol decomposition (Fig. 5) is more complicated. The temperature dependencies of catalytic activity of the samples, annealed below 973 K, pass through a maximum at about 500–525 K with a tendency to further increase above 600 K. The conversion curves for the samples, prepared at higher temperature, are much simple, but significantly shifted to higher temperatures. The main registered products in all cases are CO and methane, but CO2 (up to 12%), methyl formate (1–4%) and C2–C3 hydrocarbons (1–2%) are also observed as by-products. As a whole, the CO selectivity increases with the temperature increase. However we should stress on the unusual behavior of the CuFe(773) sample, where extremely low selectivity to CO and a predominant formation of methane is found at high temperatures. On the contrary to toluene oxidation, the XRD (ESI) and Mössbauer (Table 3, ESI) data of the samples after methanol decomposition demonstrate significant changes in the phase composition due to the influence of the reaction medium. Normally, the Mössbauer spectra are well fitted with three sextet components with parameters typical of iron carbide (χ-Fe5C2) and two sextets of magnetite (Fe3O4) phase. The relative part of magnetite phase passes through a maximum with the annealing temperature increase, being the only registered one for the CuFe(773) sample. Taking into account these results, we assume that the final, active state of the catalysts forms during some competitive redox transformations under the reaction medium that in turn, depends in a high extent on the samples initial phase composition. Transformation of copper ferrite to metallic Cu, magnetite and carbide probably occur by their interaction with the products of methanol decomposition – H2 and CO, but the reverse transformation of χ-Fe5C2 and Cu to magnetite and CuO, respectively, is not excluded due to their interaction with CO2 [22]. The combined Mössbauer, XRD and TPR measurements point to more Table 3 Parameters of Möessbauer spectra of selected samples after the catalytic tests: t – total oxidation of toluene; m – methanol decomposition. Sample

Components

IS, mm/s

QS, mm/s

Heff, T

FWHM, mm/s

G, %

CuFe(673)_t

Sx1–CuFe2O4–Fe3+octa Sx2–CuFe2O4–Fe3+tetra Sx1–CuFe2O4–Fe3+octa Sx2–CuFe2O4–Fe3+tetra Sx1–CuFe2O4–Fe3+octa Sx2–CuFe2O4–Fe3+tetra Sx1–Fe3O4 Sx2–Fe3O4 Sx3–χ-Fe5C2 Sx4–χ-Fe5C2 Sx5–χ-Fe5C2 Sx1–Fe3O4 Sx2–Fe3O4 Sx3–χ-Fe5C2 Sx4–χ-Fe5C2 Sx5–χ-Fe5C2 Sx1–Fe3O4 Sx2–Fe3O4 Sx1–Fe3O4 Sx2–Fe3O4 Sx3–χ-Fe5C2 Sx4–χ-Fe5C2 Sx5–χ-Fe5C2 Sx1–Fe3O4 Sx2–Fe3O4 Sx3–χ-Fe5C2 Sx4–χ-Fe5C2 Sx5–χ-Fe5C2

0.36 0.30 0.37 0.28 0.37 0.28 0.28 0.69 0.24 0.22 0.19 0.29 0.66 0.26 0.21 0.19 0.30 0.67 0.29 0.67 0.25 0.21 0.20 0.30 0.67 0.25 0.20 0.19

−0.08 0.01 −0.11 0.02 −0.10 0.01 0.00 0.00 0.04 0.01 0.02 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.05 0.01 0.02 0.00 0.00 0.04 0.01 0.03

51.1 47.1 51.2 48.4 50.9 48.2 49.8 46.5 21.7 19.5 11.4 50.3 46.4 22.0 19.0 11.7 50.1 46.9 49.9 46.7 22.0 18.9 11.9 50.1 46.9 22.1 19.2 11.8

0.50 0.98 0.80 0.54 0.58 0.50 0.31 0.36 0.45 0.43 0.52 0.45 0.47 0.40 0.49 0.41 0.36 0.44 0.36 0.43 0.41 0.48 0.33 0.34 0.37 0.40 0.48 0.41

18 82 46 54 43 57 5 8 34 30 23 4 5 34 38 19 35 65 13 25 23 30 9 10 17 28 30 15

CuFe(973)_t CuFe(1073)_t CuFe(573)_m

CuFe(673)

Toluene conversion, mol.%

100

CuFe(973) CuFe(573)

80

CuFe(673)_m

CuFe(773)

CuFe(1073)

60

CuFe(773)_m 40

CuFe(973)_m 20

0 500

600

700

Temperature, K Fig. 4. Total oxidation of toluene on various samples; toluene partial pressure – 0.9 kPa, WHSV-1.2 h− 1.

CuFe(1073)_m

T. Tsoncheva et al. / Catalysis Communications 12 (2010) 105–109

a CuFe(1073) CuFe(573)

Conversion, mol%

100

80

CuFe(773) CuFe(973)

60

CuFe(673)

40

20

0 400

500

600

700

Temperature, K

b 100

CuFe(673)

109

but on the phase transformations due to the influence of the reaction medium as well. Under the oxidation atmosphere, that exists during the total oxidation of toluene, the catalytic activity of the samples decreases with the annealing temperature increase and correlates with the increase of the relative part of the obtained tetragonal ferrite. Under the reduction atmosphere that realizes during the methanol decomposition, complex phase transformation with the formation of various copper (Cu, CuO) and iron (magnetite, carbide) phases is observed. The relative part of the obtained components strongly depends on the initial phase composition of the samples and reflects at the complex course of the conversion curves and at the specific changes in the selectivity to CO and methane as well. Acknowledgement Financial support of the Bulgarian Academy of Sciences and the National Science Fund of Bulgaria through projects DO 02-295/2009 and Rila4-412 (DO 02-29/03.12.2008) is acknowledged.

CO selectivity, mol%

80

CuFe(573)

Appendix A. Supplementary data

60

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2010.08.007.

CuFe(973) 40

CuFe(1073)

References

20

CuFe(773) 0 500

600

700

Temperature, K Fig. 5. Methanol decomposition (a) and CO selectivity (b) vs temperature on various samples; methanol partial pressure −1.57 kPa, WHSV-1.5 h−1.

difficult reduction of tetragonal ferrite phase with the predominant formation of Cu and χ-Fe5C2 for the materials, obtained at higher annealing temperature. In accordance with our previous study [23], the obtained final phase composition of the samples is well fitted with their high selectivity to CO (Fig. 5b). Just the opposite, the lower occupation of spinel octahedral positions with Cu2+ ions, as it is the case of CuFe (773) sample, leads to its easier reduction to metallic Cu and χ-Fe5C2 at low temperatures, that reversibly transforms respectively to CuO and magnetite with the temperature increase (Table 3, ESI). This assumption well correlates with the observed decrease in the CO selectivity and a predominant formation of methane above 600 K for this material [23]. 4. Conclusion Nanosized copper ferrite is prepared by thermal treating of the corresponding hydroxide carbonate precursor. The phase composition of the obtained product strongly depends on the annealing temperature, with a tendency to tetragonal structure formation over the cubic one at higher temperatures. The catalytic behavior of the obtained ferrites depends not only on their initial phase composition,

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